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Clicking on the links I find on the web it seems many authors don't express them neatly enough for me or they're even confusing the two. Can someone please clear this up?

Is frequency modulation

\begin{align} &\rm \sin((frequency+modulator)time-phase)\\ &\qquad\quad\textrm{or is it}\\ & \rm \sin(frequency*time+modulator-phase)\quad ? \end{align}

Ugh!

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  • $\begingroup$ Why 'carrier='? The carrier should be without modulation! $\endgroup$
    – N74
    Aug 1, 2016 at 20:36
  • $\begingroup$ @N74 Fixed and done. I think you can edit questions if you have enough points. $\endgroup$
    – user23148
    Aug 1, 2016 at 20:39
  • $\begingroup$ $x(t) = A(t) e^{i \phi(t)}$ : $A(t)$ is the time-varying amplitude, $\phi(t)$ the phase, $\omega(t) = \phi'(t)$ the time-varying (angular) frequency. for $x(t)$ to be a pitched sound, $A(t)$ has to be band-limited, and $\frac{\omega'(t)}{\omega(t)}$ too $\endgroup$
    – user1952009
    Aug 1, 2016 at 20:47
  • $\begingroup$ @user1952009 Am I 100% correct in assuming $phase modulation=sin(frequency*time-(phase+modulator))$. Can I check that off the list? $\endgroup$
    – user23148
    Aug 1, 2016 at 20:56
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    $\begingroup$ to be clear, AM is when the signal is in $A(t)$ and $w(t)$ is constant, and $FM$ is when the signal is in $\omega'(t) / \omega(t)$, and $A(t)$ is constant. phase modulation is almost the same as FM but high-pass filtered, with the signal in $\phi(t)$. AM is the most obvious way for transmitting a signal, while FM is the most robust for the radio when there are some interferances $\endgroup$
    – user1952009
    Aug 1, 2016 at 22:20

1 Answer 1

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If $t$ is time, $s(t)$ is the (appropriately scaled) signal, $\omega_0$ is the angular frequency, and $\phi_0$ is a phase offset, then phase modulation is $$ t \mapsto \sin(\omega_0 t + \phi_0 + s(t)) $$ Different signs for $s(t)$ and/or $\phi_0$ may be used, depending on conventions and context.

Frequency modulation is more complex to write down, because we want $s(t)$ to vary the derivative of the argument to the sine as a function of time. We get something like: $$ t \mapsto \sin\bigl(\phi_0 + {\textstyle\int_0^t(\omega_0+s(u))du} \bigr) $$ which is the same as $$ t \mapsto \sin\bigl(\omega_0t + \phi_0 + {\textstyle\int_0^t s(u)du} \bigr) $$

We see that phase modulation is the same as frequency modulation by the derivative of the signal -- which looks like a boost of high frequencies and attenuation of low frequencies.


Your proposed $$ t \mapsto \sin\bigl( (\omega_0+s(t))t + \phi_0 \bigr) $$ won't work -- it would look like phase modulation, with a modulation strength that increases monotonically and without bound depending on how long it is since $t=0$.

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  • $\begingroup$ How would you plot carrier sin(t) with frequency modulator that's also a sine function here desmos.com/calculator? $\endgroup$
    – user23148
    Aug 1, 2016 at 23:50

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